/Engine/Plugins/Enterprise/VariantManagerContent/Source/VariantManagerContent/Public/VariantSet.h
Sets the thumbnail from the active editor viewport. Doesn't do anything if the Editor is not available.
[UFUNCTION](Programming/UnrealArchitecture/Reference/Functions)(BlueprintCallable, Category="VariantSet|Thumbnail", Meta=(CallInEditor="true")) void SetThumbnailFromEditorViewport()